Luminescence methods, especially fluorescence spectroscopy, are uniquely suited for studying certain biomedical problems. The areas investigated were: 1. The activation and inhibition of phospholipase activity, as studied by a dye-liposome fluorescence assay. 2. The toxicity of retinoic acid analogs, as studied by fluorescence polarization of a membrane probe. 3. Energy transfer between different sites on serum albumin. 4. The kinetics of labeling of proteins and amino acids by the o-phthaldialdehyde reagent.